1. Field of the Invention
The present invention relates to a precision system for machining symmetrical surfaces generated from conic sections to a mirror finish.
2. Description of the Prior Art
In the field of modern precision machinery, it has been desired to develop a machine tool having a machining precision of smaller than 0.1 micron as required for laser technology and super-LSI technology. With conventional machining tools, however, machining precision of smaller than 0.1 micron could be achieved chiefly by grinding, and objects which could be ground have been limited to flat surfaces and circumferential surfaces such as the periphery of cylinders. With conventional machine tools, therefore, it was not possible to machine surfaces generated from conic sections, such as parabolic surfaces and hyperboloids, to a mirror finish.
The reasons for this inability with conventional machine tools could be attributed to both, errors produced by the changing temperature of machine parts of the machine tool and to problems with the system for driving the tool, particularly insufficient precision of the guide surface, poor resolution of the motor, insufficient precision of the feed screw, insufficient precision of the detector for position feedback, and inadequate response characteristics of the drive system. Plane surfaces and circumferential surfaces are machined fundamentally by controlling the relative movement between a grinder that moves only along a single axis and the workpiece. In machining surfaces generated from conic sections, however, control must be effected along the directions of two axes simultaneously.
In conventional feed drive systems, if precision is not critical, the two axes (X,Y) have been controlled simultaneously by a numerical control (NC) machine tool. With NC machine tools, however, the detection of table movement or the like is precise to 1 micron on the average, and is precise to only about .+-.0.5 micron even in particularly carefully designed machines. Furthermore, if a minimum resolution instruction value is set to 0.01 (micron/pulse) and a maximum feed rate for cutting to 600 mm/minute, pulses must be distributed at high speeds, on the order of 1 MHz, so that translating commands into control pulses (interpretation) for NC machine tools becomes difficult.
With regard to the position detector for controlling the feedback, although the amount of movement can be measured fairly accurately, only a measuring device employing a laser can now offer the precision of about 0.01 micron. Even when the laser-type measuring device is employed, however, the drive system is not capable of responding to error signals that are produced.